Mitochondrial electron transport requires the transfer of electrons from NADH or succinate to dioxygen. Obligate intermediates in this aerobic oxidation-reduction pathway are cytochrome c and cytochrome aa3. Electrons are transferred from reduced cytochrome c to cytochrome aa3 which in turn reduces dioxygen forming two water molecules. Energy is also conserved in the overall transfer mechanism resulting in the formation of ATP. The objective of the proposed research is the study of the molecular aspects of cytochrome c interaction with cytochrome aa3 from a protein chemical viewpoint. There are a number of aspects in the structure-function relationships of the cytochrome c - cytochrome aa3 -dioxygen reactions that a protein chemical approach should help define more clearly. These are (a) the minimal functional oligomeric form of cytochrome aa3, (b) the number and type of subunits composing the protomer form, and (c) the nature of and factors influencing the interaction of cytochrome c with cytochrome aa3. Using a solubilized lipid depleted cytochrome aa3 preparation the proposed approach includes characterization of the enzyme complexes by means of chromatography, electrophoresis, activity measurements, and spectroscopic methods. Direct measurement of cytochrome c interaction with the purified cytochrome aa3 is proposed. Factors influencing protein-protein interaction such as ionic strength, phospholipid levels, and other ions will be studied.